Some ink printing devices use a single printhead, but many use a plurality of printheads to increase the rate of printing. For example, four printheads may be arranged in two rows with each row having two printheads. The two printheads in the first row are separated by a distance corresponding to the width of a printhead. The first printhead in the second row is positioned at a location corresponding to the gap between the two printheads in the first row and the last printhead in the second row is separated from the first printhead in the second row by a distance corresponding to the width of a printhead. This arrangement is called a staggered full width array (SFWA) printhead assembly and an embodiment of a SFWA assembly is shown in FIG. 5.
Synchronizing the passage of an image receiving member with the firing of the inkjets in the printheads enables a continuous ink image to be formed across the member in the direction perpendicular to the direction of member passage. Alignment of the ink drops ejected by the printheads, however, may not be as expected. Each printhead in the SFWA has six degrees of positional freedom, three of which are translational and three of which are rotational. The printheads need to be precisely aligned to provide a smooth transition from the ink drops ejected by one printhead to the ink drops printed by the other printheads in the assembly. Misalignment of printheads may occur from, for example, printheads failing to meet manufacturing tolerances, thermal expansion of the printhead and associated parts of the printer, vibration of the printhead, or the like.
Misalignments between printheads in three of the six degrees of freedom may be categorized as roll or stitch errors. Roll errors can occur when a printhead rotates about an axis normal to the imaging member. Roll error causes a skew in the rows of ink drops ejected by the printhead relative to the imaging member. This skew may be noticeable at the interface between two printheads and may cause an objectionable streak. Stitch errors occur from shifts in one printhead compared to another printhead. Y-axis stitch errors arise from shifts that cause ink drop rows from the shifted printhead to land above or below the ink drop rows ejected by preceding or following printhead. X-axis stitch errors arise from shifts that cause the first and last drops in the rows printed by the shifted printhead to be too close or too far from the last and first drops, respectively, in the rows printed by the preceding and following printheads, respectively. Of course, if the shifted printhead is the first or last printhead in the assembly, shifting of the first drop or the last drop in the rows, respectively, does not occur at an intersection with another printhead. Thus, aligning printheads in a SFWA with sufficient accuracy to allow high image quality is desired.